There is high demand for good quality LED lamps to replace incandescent light bulbs. However, while their energy efficiency is widely-touted, current LED lamps suffer from a number of disadvantages in which the driver electronics may play a part:                poor light quality—while LED selection and optical design are very relevant in this regard, the driver electronics impose limitations on (i) visible flicker, (ii) invisible flicker, and/or (iii) dimming performance. In general, light quality is frequently considered to be inferior to that of incandescent bulbs, in particular when using typical AC dimmer controls;        high cost—a good quality replacement LED lamp costs in the region of £20, a clear disincentive to consumers. Although the LED chip and optical and mechanical parts form the major part of the bill of materials, the electronics are presently too expensive;        disappointing lifetime—the main argument used in justifying the high asking price of LED lamps is that they last many times as long as incandescent lamps. This originates in the long lifetimes of LED chips themselves, however some of the driver electronics parts tend to have shorter lifetimes than the LEDs. In particular, very long lifetime electrolytic capacitors—especially those rated to high voltages (˜450 V)—are either unavailable or are very expensive; and/or        poor power factor—concerns over power quality have prompted the adoption of regulations for minimum power factor for LED lamps, e.g. EC 1194/2012. This effectively prevents the use of driver circuits that store considerable energy in large, high voltage capacitors on their primary (input) side, as used in many power supplies and chargers having poor power factor.        
Many of these same disadvantages have been previously encountered in the development of fluorescent lamps, in particular that of dimmable compact fluorescent lamps (CFLs). However, due to the differing drive requirements of LEDs and fluorescent tubes, some remain to be overcome.
Regarding LED lamp performance when used with AC dimmers, it is noted that most existing dimmer controls were designed for incandescent lamps. LED lamps present a very different load to the dimmer circuit from that presented by incandescent lamps. Of particular note are the most widespread dimmer types, which employ leading edge phase modulation by triac-based circuits. In this regard, the following phenomena are of note when LED lamps are combined with phase modulating dimmers: visible pulsing of LEDs, audible noise from LED light bulbs, LEDs never fully turn on, LEDs turn on when the dimmer is turned off, and/or substantial inrush current when a triac turns on, potentially damaging both the triac and LED drivers.
Thus, there remains a need for improvements in solid state driver circuits and, in particular, LED driving, in relation to, inter alia, light quality, cost, power factor and/or lifetime, etc., for example with specific regard to ringing.
For use in understanding the present invention, the following disclosures are referred to:                R and et al, Issues, Models and Solutions for Triac-Modulated Phase Dimming of LED Lamps Power Electronics Specialists Conference (PESC) 2007;        U.S. Pat. No. 6,043,611 (Philips);        U.S. Pat. No. 5,559,395 (Philips);        U.S. patent application Ser. No. 11/445,473, issued as U.S. Pat. No. 7,567,445;        U.S. patent application Ser. No. 12/405,618, issued as U.S. Pat. No. 7,944,722; and        U.S. patent application Ser. No. 12/752,611, issued as U.S. Pat. No. 8,446,746.        